Field of the Invention
The invention lies in the semiconductor technology field. More specifically, the present invention relates to a II-VI semiconductor component having at least one junction between a layer containing Se and a layer containing BeTe, and to a process for producing the junction. It relates, in particular, to a low-loss contact with a p-conducting mixed crystal based on II-VI semiconductor materials such as, for example, ZnSe.
A plurality of processes are used for the production of electrical contacts on II-VI semiconductor components. In particular, considerable difficulties are encountered in this regard for such components made of mixed crystals based on ZnSe.
It has been found that, in the production of contacts on p-conducting ZnSe, it is inappropriate to use a simple metal contact. Due to the very high valence band affinity of ZnSe a simple metal contact will always lead to the formation of a high Schottky barrier at the metal/p-type semiconductor junction. Positive charge carriers can tunnel through such a high Schottky barrier only with difficulty.
The tunneling effect can be amplified by increasing the doping of the p-type ZnSe using very low growth temperatures, and thereby narrowing the potential barrier (cf. J. Qiu et al., Journal of Crystal Growth 12 (1993), p. 279 et seq.). Other attempts to increase the conductivity through the p-type contact on ZnSe make use of a highly conductive HgSe layer which is fitted between the metal contact and the p-type ZnSe (cf. Y. Lansari et al., Applied Physics Letters 61 (1992), pages 2554 et seq.).
Increased edge doping can also take place using a near-surface p-type ZnTe layer, which also has lower valence band affinity (corresponds to higher valence band energy) than ZnSe. A flatter Schottky barrier is therefore formed. Holes can overcome the flatter Schottky barrier more easily when the acceptor concentration is high.
Lowering the valence band affinity at the surface in order to improve the ohmic contact properties runs up against the problem that, owing to the different valence band affinity of the component cover layer (for example based on ZnSe), and of the superficial contact layer (consisting, for example, of ZnTe or BeTe), a barrier is created for holes in the semiconductor body which makes transport through the semiconductor contact structure more difficult.
It is known that the valence band discontinuity of isovalent interfaces, for example ZnSe/ZnTe or GaAs/AlAs, can only be adjusted to a small extent (cf. R. G. Dandrea, C. B. Duke, Journal of Vacuum Science and Technology B 10(4) (1992), page 1744). There is accordingly as yet no known method by which the band discontinuity at a junction such as, for example, from ZnTe to ZnSe or from BeTe to ZnSe can be reduced and the charge carrier transport thereby facilitated. With the large valence band discontinuity between BeTe and ZnSe, about 1.2 eV, or between ZnTe and ZnSe, about 0.8 eV, it is not possible to use processes for overcoming the potential barrier as are described, for example, in F. Capasso et al., Journal of Vacuum Science and Technology B 3(4) (1985), pages 1245-51, or in H. J. Gossmann et al., Critical Reviews in Solid State and Materials Science 18(1) (1993), pages 1-67.
For this reason, contact layer sequences have been proposed in which the valence band energy near the surface is gradually increased by applying semiconductor multilayers, so that the valence band edge jump is flattened and the hole barrier between ZnSe and ZnTe or between ZnSe and BeTe is lowered. For example, it has been proposed to use ZnSe/ZnTe multilayers, by means of which the valence band energy of the ZnSe within a near-surface region is raised to the valence band energy of ZnTe, and it is thus possible to produce a contact with a small Schottky barrier which has low impedance, and in particular ZnTe can be produced with high p-type conductivity (cf. WO94/15369 and Y. Fan et al., Applied Physics Letters 61 (1992), pages 3161 et seq.). This structure is referred to in the literature as xe2x80x9cgradingxe2x80x9d or xe2x80x9cpseudogradingxe2x80x9d. A similar contact structure employs the material BeTe instead of ZnTe in BeTe/ZnSe multilayers as a p-type contact, as described in WO94/15369, in P. M. Mensz, Applied Physic Letters 64(16) (1994), page 2148 or in U.S. Pat. No. 5,422,902. This is expected to give the contact layer an increased crystalline quality, which prevents lattice defects which are detrimental to the operation of a component. With the facility of producing lattice-matched BeTe/ZnSe xe2x80x9cpseudogradingxe2x80x9d heterostructure contacts with good structural quality, the p-conducting electrical connection to a II-VI component can also be put onto the interface with a p-conducting substrate, as discussed in WO94/15369.
The described pseudograding contact has a complicated structure with many internal interfaces. It consists of a multilayer in which BeTe alternates with ZnSe, the layer thickness proportions of these two components being varied gradually. The total thickness of the ZnSe/BeTe contact layer sequence is between 200 xc3x85 and 1000 xc3x85 (cf. P. M. Mensz, Applied Physic Letters 64(16) (1994), page 2148 or U.S. Pat. No. 5,422,902). Over this length, the average BeTe concentration is adjusted in steps from 0 to 100% by increasing the BeTe layer thickness while at the same time reducing the ZnSe layer thickness. With the usual production parameters for BeTe and ZnSe, the total resistance of the BeTe/ZnSe contact structure is in the range from 10xe2x88x922 to 10xe2x88x923 Wcm2, and therefore leads to high electrical losses in the contact structure. The high resistance is attributed to the large valence band discontinuity between BeTe and ZnSe, which is 1.21 eV.
There arises another problem in the context of the contact structures with ZnSe/ZnTe or ZnSe/BeTe multilayers (pseudograding), namely when the concentrations of the dopant nitrogen are high, a large number of lattice defects are produced and make it possible for the superlattice matrix elements and the dopant to interdiffuse. Such defects can also aggregate and produce extended lattice defects in contact, which can greatly disrupt the operation of a laser diode and therefore shorten its life.
Another problem of the BeTe/ZnSe pseudograding contact is that, with the layer thicknesses used, ripples occur on the interfaces between BeTe and ZnSe which are created in order to reduce the elastic prestresses. This unevenness also has a negative effect on the functioning of a laser diode.
The transfer of holes from the valence band of, for example, BeTe to the valence band of ZnSe occurs not only in contact structures, but is also relevant, for example, in vertically emitting lasers with Bragg reflectors that contain BeTe and ZnSe. For such component structures, the use of extended grading or pseudograding is highly disadvantageous.
It is accordingly an object of the invention to provide a II-VI semiconductor component with an improved junction from an Se-containing layer (e.g., a ZnSe layer) and a BeTe-containing layer, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type. It is a further object to provide a simple process for reproducible production of the junction.
It is a particular object to provide a contact in which the valence band discontinuity at an interface between a layer containing BeTe and a layer containing ZnSe is greatly reduced, and low-loss transfer of holes from the layer containing BeTe to the layer containing ZnSe can be achieved, in particular for p-conducting layers. The intention is especially to provide contact structures with which it is possible to produce low-loss p-type contact with p-conducting II-VI semiconductor layers that contain ZnSe.
The term materials containing BeTe is used below to denote materials such as, for example, BexMgyZn1xe2x88x92xxe2x88x92yTe, BexCdyZn1xe2x88x92xxe2x88x92yTe, BexMgyCd1xe2x88x92xxe2x88x92yTe, BexMnyZn1xe2x88x92xxe2x88x92yTe, BexSryZn1xe2x88x92xxe2x88x92yTe, BexBayZn1xe2x88x92xxe2x88x92yTe (0x1, 0y1, x+y1) or other mixed crystals, that contain Be and Te or tellurium.
The term materials containing ZnSe is used below to denote materials such as, for example, BexMgyZn1xe2x88x92xxe2x88x92ySe, BexCdyZn1xe2x88x92xxe2x88x92ySe, BexMgyCd1xe2x88x92xxe2x88x92ySe, BexMnyZn1xe2x88x92xxe2x88x92ySe, BexSryZn1xe2x88x92xxe2x88x92ySe, BexBayZn1xe2x88x92xxe2x88x92ySe (0x1, 0y1, x+y1) or ZnxMg1xe2x88x92xSySe1xe2x88x92y or BexZn1xe2x88x92xSySe1xe2x88x92y (0 less than =x less than =1, 0 less than =y less than =1) or other mixed crystals, that contain Zn and Se or selenium.
With the foregoing and other objects in view there is provided, in accordance with the invention, a II-VI semiconductor component, which comprises a layer sequence including a semiconductor layer containing BeTe and a semiconductor layer containing Se and forming a junction therebetween with an interface, wherein the interface between the semiconductor layer containing BeTe and the semiconductor layer containing Se within the layer sequence is prepared such that a Bexe2x80x94Se bond configuration is formed.
In other words, according to the invention, within a layer sequence, e.g. a contact region, on an active layer sequence of an optoelectronic II-VI semiconductor component (light-emitting or light-receiving), at least one semiconductor layer containing BeTe is applied epitaxially (e.g. using MBE or MOCVD) on at least one semiconductor layer containing Se, in particular containing ZnSe, or at least one semiconductor layer containing Se is similarly applied on at least one semiconductor layer containing BeTe, and an interface between the semiconductor layer containing BeTe and the semiconductor layer containing ZnSe is prepared, in particular by introducing an Se-rich interlayer, in such a way that a Bexe2x80x94Se configuration is formed, which represents a potential barrier (valence band discontinuity) for holes of less than 1.2 eV.
This interfacial configuration is produced according to the invention by always ending or starting the epitaxial growth of ZnSe with Se coverage and/or growing ZnSe under Se-rich conditions.
In accordance with an added feature of the invention, the potential barrier for holes is less than 0.4 eV.
In accordance with an additional feature of the invention, a junction layer formed of an alloy with graded variation in composition is disposed between the semiconductor layer containing Se and the semiconductor layer containing BeTe.
In accordance with a preferred embodiment, the graded alloy is a digital graded alloy.
In accordance with another feature of the invention, at least one layer delta-doped with acceptors is inserted between the semiconductor layer containing Se and the graded alloy or digital graded alloy.
In accordance with again another feature of the invention, at least one layer delta-doped with donors is disposed between the graded alloy or digital graded alloy and the semiconductor layer containing BeTe.
In accordance with again an added feature of the invention, at least one layer doped with acceptors is inserted in the semiconductor layer containing Se, at a given distance from the interface between the semiconductor layer containing BeTe and the semiconductor layer containing Se.
In accordance with a related feature of the invention, at least one layer doped with acceptors is inserted in the semiconductor layer containing BeTe, at a given distance from the interface between the semiconductor layer containing BeTe and the semiconductor layer containing Se.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing the above-outlined II-VI semiconductor component, which comprises epitaxially growing the layer containing Se and ending or starting the epitaxial growth with Se coverage and/or growing the layer containing Se under Se-rich conditions.
In accordance with yet an added feature of the invention, an Se flux is applied to the surface of the layer containing Se before the growth of the layer containing BeTe.
In accordance with a concomitant feature of the invention, the Bexe2x80x94Se configuration is prepared by growing the layer containing Se under excess Se.
In a preferred process for the interface preparation according to the invention, the configuration at the interface is achieved by, after having grown a ZnSe layer and before growing a BeTe layer, applying an Se flux with a beam-equivalent pressure between 1xc3x9710xe2x88x925 and 1xc3x9710xe2x88x928 torr to the surface of this ZnSe layer, e.g. for from 0.5 to 60 seconds. The substrate temperature is in this case between 150xc2x0 C. and 350xc2x0 C., but preferably between 200xc2x0 C. and 250xc2x0 C.
The Se surface stabilization may be followed by a growth pause of up to 20 seconds, but preferably less than 5 seconds, although this may be omitted.
On the Se-stabilized ZnSe/Se surface produced in this way, which can be identified by clear (2xc3x971) surface reconstruction in RHEED (reflection high energy electron diffraction) measurements, the BeTe layer is applied in such a way that, during the growth of the BeTe layer, the flux ratio between Te and Be is between 2 and 50, and a Te:Be ratio is preferably set between 2 and 10. Following the BeTe growth, the growth is interrupted for from 0 to 180 seconds, preferably between 1 and 10 seconds. During this, the substrate temperature is maintained at the ZnSe growth temperature of from 200xc2x0 C. to 250xc2x0 C., although it may also be increased up to 550xc2x0 C. Following the interruption to the growth, Se is applied on the surface at substrate temperatures of between 150xc2x0 C. and 350xc2x0 C. for from 0 to 180 seconds. It is then possible to start growing ZnSe on a BeTe/Se surface prepared in this way.
In another preferred variant of the method according to the invention, the Bexe2x80x94Se configuration of the interface is prepared by growing ZnSe with excess Se (Se:Zn ratio 1.1:1 to 5:1), preferably at substrate temperatures of between 150xc2x0 C. and 350xc2x0 C. The ZnSe growth rate is then advantageously between 0.1 and 1 monolayer. Such growth conditions lead to a clear (2xe2x80x21) reconstruction on the ZnSe surface. In this case, there need not necessarily be a growth interruption at the interfaces.
The effect achieved with the described process is that, at the interface between BeTe and ZnSe, a bond is formed between Be and Se, but not between Zn and Te. In this case, use is made of the displacement of Te atoms, which are in a bound state on the BeTe surface, by Se atoms. Conversely, displacement of Se by Te does not take place on the ZnSe surface. The advantage of the interface with Bexe2x80x94Se configuration is that the valence band discontinuity between layers containing BeTe and layers containing ZnSe is reduced from above 1.2 eV to about 0.4 eV. In novel contact structures, for example of BeTe and ZnSe, this remaining potential barrier for holes in the valence band can be reduced further by controlled doping with acceptors and donors.
The contact structure according to the invention is, in particular, intended for an optoelectronic component on a substrate made of a material from the group consisting of GaAs, InAs, InGaAs, GaP, InP, Si, Ge, ZnO, ZnSe, ZnTe, CdTe, ZnCdTe having an active layer intended to produce radiation, in which the active layer is designed as a quantum well or superlattice with predetermined period, or multiple quantum well or quantum dot structure, in which the active layer is arranged between layers that are doped for electrical conductivity with the opposite sign to one another, and in which there are electrical contacts that consist of a sequence of materials containing selenium and layers containing BeTe, and there is an interface between the layers belonging to these material groups in the contact layer sequence, and this interface is prepared in such a way as to create an interfacial organization at the material junction with a valence band discontinuity of less than 1.2 eV.
The material containing ZnSe is in this case preferably from the group BexMgyZn1xe2x88x92xxe2x88x92ySe, BexCdyZn1xe2x88x92xxe2x88x92ySe, BexMgyCd1xe2x88x92xxe2x88x92ySe, BexMnyZn1xe2x88x92xxe2x88x92ySe, BexSryZn1xe2x88x92xxe2x88x92ySe, BexBayZn1xe2x88x92xxe2x88x92ySe (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6y less than 1, x+yxe2x89xa61), ZnxMg1xe2x88x92xSySe1xe2x88x92y, BexZn1xe2x88x92xSySe1xe2x88x92y (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), and the layer containing BeTe from the group BexMgyZn1xe2x88x92xxe2x88x92yTe, BexCdyZn1xe2x88x92xxe2x88x92yTe, BexMgyCd1xe2x88x92xxe2x88x92yTe, BexMnyZn1xe2x88x92xxe2x88x92yTe, BexSryZn1xe2x88x92xxe2x88x92yTe, BexBayZn1xe2x88x92xxe2x88x92yTe (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, x+yxe2x89xa61), BexZn1xe2x88x92xSyTe1xe2x88x92y (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), BexCd1xe2x88x92xSyTe1xe2x88x92y (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), BexMg1xe2x88x92xSySe1xe2x88x92y (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
The interfacial organization with a small valence band offset is a Bexe2x80x94Se configuration at the interface. The latter is preferably formed by, during the production of the semiconductor layers, covering the layers containing ZnSe and/or the layers containing BeTe with selenium from a selenium particle flux at the interface which is formed.
In one refinement, at least one layer with increased acceptor concentration is inserted in the layer containing ZnSe, at a distance d from the interface between the layer containing BeTe and the layer containing ZnSe. The distance d is preferably smaller than 40 xc3x85.
A layer with increased sheet charge density may contain tellurium or BeTe. Each layer with increased sheet charge density is, for example, between 0.5 monolayer and 5 monolayers thick. Instead of the layer containing BeTe, a metal contact may be applied directly on the layer containing ZnSe. A layer with increased acceptor concentration may be placed at a distance d1 from the interface in the layer containing ZnSe, and a layer with donors may be placed at a distance d2 from the interface in the layer containing BeTe. The distances d1 and d2 may be of different or equal value. The acceptor concentration or donor concentration in the heavily doped layers may be of different or equal value. The distance d1+d2 is, for example, between 5 xc3x85 and 300 xc3x85. The level of the sheet charge densities in the heavily doped layers may be more than 1012 cmxe2x88x923, and the thickness of the heavily doped layers is, for example, no more than 50 monolayers. The area doped with donors contains, for example, selenium or ZnSe. Al, Cl, Br or iodine may be used as donor dopant. The area heavily doped with acceptors contains, for example, Te or BeTe and, for example, N, As, Sb, P or another element from main group I, IV or V is used as acceptor dopant.
A junction layer, which consists of an alloy with graded variation in composition, may be placed between a layer containing ZnSe and a layer containing BeTe.
All the layers containing ZnSe and BeTe may be doped with p-type conductivity. The alloy may be formed as a digital alloy. The variation in the composition of the graded or digital graded alloy is linear or nonlinear. At least one delta-doped layer with an increased acceptor concentration may be inserted between the layer containing ZnSe and the graded or digital graded alloy. At least one delta-doped layer with donors may be placed between the graded or digital graded alloy and the layer containing BeTe. Al, Cl, Br or I may be used as donor dopant. The delta-doped layers are, for example, between 0.5 and 50 monolayers thick. The sheet charge density in the layers delta-doped with acceptors and donors may be the same, and adjusted as a function of the separation of the delta-doped layers in such a way that the internal electric field of the doping dipole which is created compensates for the potential gradients within the graded or digital graded alloy. The separation of the delta-doped layers is, for example, less than 300 xc3x85.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a II-VI semiconductor component having at least one junction between a layer containing se and a layer containing BeTe, and process for producing the junction, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.